How can I replicate an object over the surface of another?
One way to go about this would be to use DupliFaces.


  Duplication Faces or DupliFaces is the capability to replicate an
  object on each face of a parent object.


You would model your nail then parent it to the cactus object with Ctrl + P. You then select the cactus object and in the Object panel under Duplication and choose Faces.

This is not perfect and the outcome of this depends a lot on the topology of your parent object (the cactus) and how your child object (the nail) is scaled and or rotated etc. It can work with some tweaking however&hellip;



Another way would be to use a particle system, set the type to Hair and under Render, select the nail object. This is a bit harder to tweak if you don't have much experience with it etc. but can give you alot more control and variation as to how things can be placed.